Nonuniform quasiparticle densities within fluxonium qubit

Excess quasiparticles densities in superconducting qubits – far exceeding thermal equilibrium values predicted by Bardeen-Cooper-Schrieffer (BCS) theory – constitute an adverse decoherence channel. Moreover, quasiparticle bursts from high-energy radiation impacts significantly limit the performance of fault-tolerant quantum error correction schemes. While transmon qubits have served as the main platform for studying quasiparticle effects, fluxonium qubits remain far less explored in this context. Here, we investigate energy relaxation in fluxonium qubits under controlled on-chip quasiparticle injection. By accounting for the non-stationary dynamics of quasiparticle relaxation, we extract instantaneous energy-relaxation rates that allow us to disentangle quasiparticle-induced decoherence in the single Josephson junction and the junction array. Remarkably, we uncover a striking, order-of-magnitude disparity in quasiparticle density, with quasiparticle density being consistently higher near the single junction than in the array. This imbalance persists across fluxonia with different energy spectra and in both photon- and phonon-dominated regimes, suggesting intrinsic qubit properties as the governing factor of quasiparticle dynamics. The findings offer insight for reconciling previously reported discrepancies in bounds on quasiparticle densities in fluxonium circuits.